1. Field of the Invention
The present invention relates to an absolute, batch processible solid-state oxygen microsensor for use in applications such as combustion systems to maintain and improve combustion efficiency levels. The solid-state oxygen microsensor of the present invention measures the potential difference (EMF) generated by two electrodes deposited on a solid oxygen ion conducting electrolyte and located in a temperature gradient in the same ambient atmosphere. This invention further relates to thin film structural configurations which allow thermal expansion forces to bend or deflect the film into an unconstrained space.
2. Description of the Prior Art
Sensors for determining the oxygen composition of gaseous mixtures, such as automotive exhaust, are well known to the art. For example, U.S. Pat. Nos. 4,272,329, 4,272,330, and 4,272,331 teach an oxygen sensor comprising a pump cell and a sensor cell, each having solid zirconia electrolyte and thin platinum electrodes attached thereto. The sensor cell and the pump cell, along with a ceramic tube, form an enclosed volume in which the ambient air establishes an equilibrium by means of a leak opening in the ceramic tube. The pump cell is connected, by external circuitry, to an electrical input, while the sensor cell is coupled, by external circuitry, to electrical output measurement and control means.
The oxygen sensor taught by the '329 patent is operated in a steady-state mode whereby voltage is applied to the pump cell to electrochemically pump oxygen from the enclosed volume until a steady-state is reached wherein the rate of oxygen pumped from the volume is in equilibrium with the rate of oxygen diffusing into the volume through the leak hole. At steady-state, the oxygen partial pressure in the enclosed volume is reduced from ambient, causing an EMF to develop across the electrodes of the sensor cell. By adjusting the pump cell current continuously to provide a constant sensor cell voltage, the pump cell current is linearly proportional to the percentage oxygen in the ambient atmosphere.
The oxygen sensor taught by the '330 patent uses a similar device operated in a transient mode to measure oxygen partial pressure. After ambient atmosphere of a desired oxygen partial pressure is established in the enclosed volume, the pump cell is activated to withdraw oxygen from the enclosed space. Reduction of oxygen partial pressure in the enclosed space causes an EMF to develop across the sensor cell. The first derivative of sensor cell voltage/time evaluated at or shortly after the initiation of a voltage drop is inversely proportional to the ambient oxygen partial pressure. The oxygen sensor may also be operated by pumping oxygen into the enclosed space and reversing the sign of the initial sensor cell voltage to determine the ambient oxygen partial pressure.
The oxygen sensor taught by the '331 patent uses a similar device operated in an oscillatory mode whereby a repetitive sequence of oxygen pumping currents flow to the pump cell in response to voltage inputs from the sensor cell. The pump cell withdraws oxygen from the enclosed space until the voltage drop induced at the sensor cell equals a predetermined reference value. The polarity of the pump cell current is then reversed to pump oxygen into the enclosed space until the sensor cell voltage reaches another predetermined reference value, at which time the pump cell current is again reversed and the cycle is repeated. With the magnitude of the pump cell current fixed, the period of oscillation is proportional to the oxygen partial pressure.
U.S. Pat. No. 4,510,036 teaches a limiting electric current type oxygen sensor with a microheater formed on an insulating film layer on one of the electrodes. The insulating film limits the amount of oxygen permeated in the electrolyte and electrically insulates the electrolyte from the heater. A small hole is provided in the insulating film to allow permeation of oxygen to the electrolyte. A constant temperature heating control circuit is preferably used to control the heater temperature in accordance with the heater resistance and the electrolyte resistance to obtain a limiting electric current type oxygen concentration detector.
U.S. Pat. No. 4,559,126 teaches an oxygen sensor comprising a plurality of electrolyte layers with electrodes mounted on two of the electrolyte layers and a plurality of ceramic layers with a heater provided in one ceramic layer and a high electric resistance ceramic layer provided between the heater layer and the solid electrolyte layers.
U.S. Pat. No. 4,502,939 teaches an oxygen sensor having electrodes in contact with a solid electrolyte, the electrodes covered with a porous sintered cover layer and a gas tight cover extending over the electrodes and the porous cover layer. Exhaust gases, or the like, are conducted to the sensing electrode, while oxygen from ambient air, or the like, is conducted to the reference electrode through the porous sintered layer. U.S. Pat. Nos. 4,505,806 and 4,505,807 teach an oxygen sensor comprising an oxygen pump cell and an oxygen concentration cell, each having electrodes in parallel alignment on opposite surfaces of solid electrolyte boards, with a ceramic intermediate board having a cavity interposed between the oxygen pump and concentration cells providing communication to the ambient atmosphere. As the oxygen sensor is selectively heated, the oxygen concentration cell measures a ratio of oxygen concentration in the cavity to oxygen concentration of the ambient atmosphere outside the sensor, while the oxygen pump provides diffusion of oxygen between the cavity and the ambient outside atmosphere.
U.S. Pat. Nos. 4,040,929 and 4,107,019 teach the use of thin film electrolytes. The '929 patent teaches a thin film electrolyte such as yttria-stabilized zirconia sputtered onto a substrate layer to form a film about 0.030 to 1.50 microns thick which provides an oxygen sensor capable of operating at temperatures below about 2000.degree. C. The '019 patent teaches the use of a thin film electrolyte supported on a non-conductive base plate and a thin film heater in a metal/metal oxide oxygen concentration cell system. U.S. Pat. No. 4,500,412 teaches an oxygen sensor with a heater layer about 0.2 to 20 microns thick formed on an insulating substrate and covered by a protective layer about 0.01 to 500 microns thick.
Several prior art patents relate to sputtering methods for depositing thin film electrode and electrolyte layers. U.S. Pat. No. 4,244,798 teaches a method for sputtering a porous, high surface area platinum film electrode onto a zirconia thimble. U.S. Pat. No. 4,253,931 teaches another sputtering process for depositing a platinum electrode onto a zirconia thimble in a specified atmosphere at specified pressures. U.S. Pat. No. 4,521,287 teaches yet another sputtering process for depositing high surface area platinum electrode films on a zirconia thimble for use as an exhaust gas oxygen sensor.
U.S. Pat. No. 4,419,213 relates to an oxygen concentration cell formed as a laminate of a plurality of thin layers, including solid electrolyte, supported on a ceramic substrate. A heater is embedded in the ceramic substrate layer and heater lead wires are insulated from concentration cell lead wires by the ceramic substrate to prevent leakage of the heater current. U.S. Pat. No. 4,450,065 teaches an oxygen sensor comprising a pump cell and an oxygen concentration cell, each having a solid electrolyte layer and electrodes deposited thereon. The two cells are coupled in parallel leaving a gap between the two cells. The oxygen concentration cell measures the ratio between oxygen concentration in the gap and oxygen concentration of gas outside the sensor, while the pump cell diffuses oxygen between the gap and the outside atmosphere. U.S. Pat. No. 4,487,680 teaches an oxygen pumping device having two electrolyte layers which may have different porosities and which contact one another, and three electrodes which function to provide both oxygen pump and sensor cells. This oxygen sensor does not require an enclosed volume and can be produced at low cost by conventional planar layer technology.
U.S. Pat. No. 4,126,532 utilizes a metal/metal oxide sinter as a reference oxygen source supported on a base member with a solid electrolyte layer and another electrode to provide an oxygen sensor in which an EMF is developed across the electrodes at relatively low temperatures. U.S. Pat. No. 4,207,159 teaches an oxygen sensor having a probe comprising a porous, conductive reference electrode adjacent a solid electrolyte layer with a similar porous separator layer on the exterior surface of the reference electrode whereby the reference electrode is in communication with the exterior atmosphere through the porous layer. During measurement, current flows through the electrolyte to maintain a reference oxygen partial pressure at the interface between the electrolyte and the reference electrode.
U.S. Pat. No. 4,326,318 teaches an oxygen sensor for measuring oxygen partial pressures which relies upon detection of a thermodynamic transition temperature which interfaces a high resistivity mode and a low resistivity mode. A variable potential is applied across two electrodes separated by a conductive electrolyte, and the resistance between the first electrode and a third electrode isolated from the second electrode is monitored to determine the thermodynamic transition temperature, from which the oxygen partial pressure in atmosphere at a known temperature is determined.
Solid electrolyte oxygen sensors are known which generate an internal oxygen reference, thereby eliminating the conventional oxygen reference. "Internal-Reference Solid-Electrolyte Oxygen Sensor", David M. Haaland, Analytical Chemistry, Vol. 49, No. 12, October 1977. A sensor cell monitors the ratio of oxygen partial pressure inside and outside the sensor, while the other cell functions as a pump. The internal oxygen reference is generated by pumping oxygen from a known cavity volume and then pumping oxygen back into the cavity until the oxygen partial pressure equilibrates.
It is known in the art that combustible gases can be sensed by hot-wire catalytic sensors and metal oxide semiconducting sensors, which change conductivity when exposed to combustible gases to H.sub.2 O, CO.sub.2 or O.sub.2, as well as by exposing a two metal semiconductor junction pair to a combustible gas in such a way that one junction having no combustion catalyst remains at a constant temperature and the other junction having a combustion catalyst experiences a small temperature rise due to released heat of combustion. Such a temperature difference output is registered and calibrated in terms of gas concentration. "Solid State Gas Sensors", P. T. Moseley and B. C. Tofield, Eds., The Adam Hilger Series on Sensors, Adam Hilger, Bristol and Philadelphia, pgs. 139-150, 1987.
Several prior art literature references known to the inventors relate to solid-state oxygen microsensor technology and materials. Researchers have demonstrated experimentally that conductivity in calcia-stabilized zirconia thin films deposited by rf sputtering is due to oxygen anion migration. "Composition, Structure, and ac Conductivity of rf-Sputtered Calcia-Stabilized Zirconia Thin Films", M. Croset, et al, Journal of Applied Physics, Vol. 48, No. 2, Feb. 1977. Fabrication of microbridges of SiO.sub.2 film over a cavity etched into a silicon chip is described in an article entitled "Microheater and Microbolometer Using Microbridge of SiO.sub.2 Film on Silicon", M. Kimura, Electronics Letters, Vol. 17, No. 2, Jan. 22, 1981. Techniques for micromachining silicon structures and fabricating thin film structures are known to provide temperature sensitive resistors which are thermally isolated from the silicon chip. Locally high temperatures may thus be achieved with very low power output and reduced heat losses to the chip. "A Microtransducer for Air Flow and Differential Pressure Sensing Applications", G. B Hocker, R. G. Johnson, R. E. Higashi, P. J. Bohrer, Workshop on Micromachining and Micropackaging of Transducers, Case Western Reserve University, Cleveland, Ohio, Nov. 7-9, 1984.
Prior art oxygen microsensors utilizing an oxygen ion conducting solid electrolyte require either providing and pumping a cavity which involves slow time constants, measuring resistance-dependent values which tend to be drifty, provision of reference gas chambers, observation of metal-metal oxide redox reactions, or complex electronic circuitry. Prior art Nernstian oxygen sensors also require that the electrodes be maintained in a constant temperature environment to avoid false signals generated by thermocouple-like operation.
Prior art oxygen sensing, utilizing the known Seebeck (i.e. thermocouple) effect, in a bulky arrangement involving electrodes at a difference in temperature, both exposed to the same oxygen concentration, is described by Pizzini et al. in "Solid State Electrochemistry II, Devices and Electrochemical Processes", La Chimica E L'Industria, Vol. 55, No. 12, p. 966, December 1973.
Existing thin film oxygen sensors having bridge or diaphragm connections to a substrate thermally expand and tend to severely deform at elevated temperatures thus often resulting in fractures.